1. Field of the Invention
The present invention relates to radiation curable inkjet inks for producing cured images exhibiting high flexibility and scratch resistance.
2. Description of the Related Art
In inkjet printing, tiny drops of ink are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a print head for ejecting the drops image-wise on an ink-receiver.
The composition of the inkjet ink is dependent on the inkjet printing method used and on the nature of the ink-receiver to be printed. UV-curable inks are more suitable for non-absorbent ink-receivers than e.g. water or solvent based inkjet inks. However the behaviour and interaction of UV-curable inkjet ink on substantially non-absorbing ink-receivers is quite complicated compared to water or solvent based inks on absorbent ink-receivers. In particular, the combination of good flexibility with good scratch resistance on a non-absorbent ink-receiver is problematic. Changes in the ink to improve the flexibility always reduce the scratch resistance and vice versa. In addition, other requirements such as good adhesion, good curing speed, controlled spreading of the ink on the ink receiver and low viscosity further limit the options for designing an inkjet ink exhibiting simultaneously good flexibility and good scratch resistance.
Increasing the ratio of polyfunctional monomers over monofunctional monomers in UV curable inks leads to improved curing speed. A high scratch resistance is obtained due to a highly cross-linked three-dimensional structure. However, the image of such an ink has a low flexibility and easily cracks on the recording medium or even peels off.
Decreasing the ratio of polyfunctional monomers over monofunctional monomers in UV curable inks leads to less crosslinked polymers, i.e. a higher flexibility, but lower curing speed and scratch resistance. The chemical nature of polyfunctional and monofunctional monomers also affects the adhesion quality and the viscosity of the inkjet ink.
In general, the approach to simultaneously enhance flexibility and scratch resistance is to use specific monomers and monomer combinations in the ink.
For example, US 2007071953 (FUJI) discloses an ink composition including a polymerization initiator; and at least one compound selected from the group consisting of (meth)acrylic acid and monofunctional (meth)acrylic acid esters and amides each having a carboxy group in a molecule. The presence of a carboxy group in the monomer provides a hydrogen-bonding interaction as shown in [0022] which is a weaker bond than the covalent bond made by polyfunctional monomers. The hydrogen-bonding bond can be broken thus allowing higher flexibility. Large amount of such acrylic acid monomers in the ink can however cause corrosion in inkjet print heads.
Another example is given by US 2004024078 (SEIREN) disclosing a UV curable ink including a coloring component, a reactive oligomer and/or a reactive prepolymer, a reactive diluent and a photoinitiator, wherein a polymer of the reactive oligomer and/or reactive prepolymer and a polymer of the reactive diluent have a glass transition point of 0° C. to 70° C. The cured film of such an ink exhibited good flexibility, scratch resistance and adhesion. However, according to [0021] the ink compositions have a rather high viscosity of 60 to 800 cps at 25° C., thus requiring high jetting temperatures of 60° or more.
US 2008108747 (FUJI) discloses ink composition including a polymerization initiator, a (meth)acrylate having a double bond with a carbon atom having an sp3 hybrid orbital at an alpha position, and a colorant. Using the specific polymerizable compound in an ink composition leads to an enhanced image flexibility after curing, while maintaining high sensitivity and adhesion of the image to a recording medium.
US 2009087576 (FUJI) discloses at [0085] that from the viewpoint of flexibility and scratch resistance of a cured material being improved, a monofunctional cyclic (meth)acrylate is preferably included in the ink composition, more preferably one of the compounds M-1 to M-29 or phenoxyethyl acrylate are used.
US 2007211111 (FUJI) discloses an ink composition including an N-vinyl lactam, a radically polymerizable compound, and a polymerization initiator, the content of the N-vinyl lactam being at least 10 wt % of the ink total weight, and the content ratio by weight of N-vinyl lactam to the radically polymerizable compound being 1:8.5 to 1. The ink leads to cured images having excellent flexibility and adhesion to a substrate.
U.S. Pat. No. 6,310,115 (AGFA) discloses radiation curable inkjet inks containing radiation curable monomers containing vinylether and acrylate functions. The vinylether acrylates can achieve low viscosity, needed in ink jet printing, without introducing a significant amount of water or other solvent.
Despite all the suggested ink compositions, there still remains a need for radiation curable inkjet inks exhibiting good flexibility and scratch resistance, while maintaining low viscosity for jetting performance, a high cure speed and a good adhesion to a wide range of substrates.